1. Field of the Invention
The present invention relates to an improved process for preparing cyclic and open-chain ketals, in particular of triacetonamine.
2Description of the Background
The ketalization of triacetonamine (2,2,6,6-tetramethyl-4-piperidone, TAA) with mono- or polyhydric alcohols to give the corresponding open-chain or cyclic ketals is known in principle from the literature. For instance, the synthesis of various TAA ketals is described, inter alia, in DE 22 03 533, DE 23 53 538, CS 272 639, U.S. Pat. No. 3,790,525, U.S. Pat. No. 3,940,401, EP 0 222 512, EP 0 291 238 and EP 0 141 502. The catalysts used are those acids known to those skilled in the art for this type of reaction, such as aqueous hydrochloric acid, sulfuric acid, phosphoric acid or various sulfonic acids (usually p-toluenesulfonic acid). Since the ketalization of TAA already uses one acid equivalent for the neutralization of the secondary amine function, a superstoichiometric amount of acid always has to be used, unlike in the case of neutral compounds. A small excess of catalyst is therefore necessary for the reaction in every case.
In addition, there are examples in the literature which do not start from TAA itself, but rather from its hydrochloride (Monatsh. Chem. 93, 1962, 1090-1106; Scripta Fac. Sci. Nat. Univ. Masaryk. Brun., Vol. 23,1993; JP 56 138 189; JP 56 025 185). This TAA hydrochloride is then converted to the ketal in a similar manner to the abovementioned examples after adding a catalyst acid and the particular alcohol. However, this procedure means an additional synthesis and isolation step, which is of little advantage for the industrial scale production of TAA ketals.
In addition, the ketalization of TAA hydrochloride and a sulfonic acid salt of TAA have also been described by reaction with an orthoester (EP 0 748 849) and by transketalization (U.S. Pat. No. 4,250,312, JP 55 092 386, EP 0 748 849).
One use of the ketals of TAA is as stabilizers in the polymer field. In addition, the corresponding N-oxyl radicals are accessible from them by oxidation of the secondary amine function, and can in turn be used as an oxidation catalyst, as polymerization inhibitors or as mass regulators in polymerizations.